*Puts on detective hat*
Ok, we've ruled out electrical, water, air, food, and pasture. Our last options are paranormal and predator.
The snake nest idea sounds very probable. I'd search for it, and lay down moth balls or some other form of snake deterrent. Call your lcal game warden, look into what animals are in your area.
Id also google your place, talk to a medium, and ask around to see about paranormalty.
” You know you’re a horse person when you walk past someone and instead of saying ‘excuse me’ you poke them in the ribs and say ‘over’ “